The invention relates to electronic distance measurement generally, and more specifically to specialized apparatus for providing indicia at a receiving location of range to an unmodulated transmitter.
It has long been well known that, for a given emitted signal strength from a radio transmitter, the signal magnitude at a receiving location decreases as the square of the distance between transmitter and receiver. Practical receivers for general purpose use always incorporate automatic gain control circuitry (AGC) so that received signal energy results in substantially the same output from the receiver.
Such AGC arrangements are particularly important where the receiver is mobile, as for example in automobile receivers, since the received radio frequency signal strength varies as distance between transmitter and receiver changes and as a result of other factors.
One particular prior art system based on the processing of a receiver AGC signal to provide distance indication at a receiving location in a personal surveillance arrangement is disclosed in German patent (Offenlegungeschrift) No. 2913563 issued Oct. 16, 1980. In that reference, a transmitted signal is demodulated, the AGC receiver signal is compared to a threshold, and if the AGC magnitude indicates a range between transmitter and receiver exceeding a threshold, an indication is given. Modulation or coding of the transmitted signal in that system is not directly related to the distance determination.
Prior art systems presently known d not appear to address the need for active distance monitoring, i.e., provision of progressively changing distance information so that a range value exceeding a predetermined threshold can be anticipated. Effective surveillance depends on trend or progressive information not provided by simple reliance on prior art systems which merely indicated that a range threshold has been exceeded.
The manner in which the invention improves and adds to the state of the prior art to produce a much improved device for the purpose will be understood as this specification proceeds.